1. Field of the Invention
This invention relates to a process and an apparatus for the vacuum treatment of metals, in particular steel.
2. Background Information
During ladle degassing, the tapping ladle filled with steel is lowered into a large cylindrical chamber, and this chamber is then sealed vacuum tight by a cover. The seal is generally a rubber ring. The covers are manufactured either of steel castings or of sheet metal structures. On the underside of the cover there is an ejector protector made of sheet metal and/or refractory tamping clay. Alloy addition devices and viewing ports are located on the cover.
The vacuum is generally generated by at least four ejectors. When molten steel is exposed to a vacuum pressure, gas bubbles form inside the steel at a pressure which is a function of the internal pressure above the surface of the bath. Unkilled steel with a high oxygen content experiences an agitation action as a result of the formation of carbon monoxide even at pressures of less than 200 Torr in the open space of the vacuum treatment vessel, and thus simultaneously flushes the hydrogen and nitrogen out of the molten metal, so that even with this relatively weak vacuum a removal of gases occurs. As the pressure is further reduced, the agitation can become vigorous, so that for example, the molten steel can rise one meter and more in the ladle. Sufficient room for the steel to rise must be provided by the selection of a rather large ladle is necessary, and therefore the ladle cannot be filled to its edge, rather it must have a certain freeboard. In existing steel mills, the ladle sizes and weights when full are determined as a function of the crane equipment. As a result of the need for the freeboard, the ladles can no longer be filled to the rim, which has the disadvantage of reducing production. The alternative solution, of enlarging the ladles, means that the hoisting apparatus and the pick-up equipment must be adjusted to the increased weight to be transported.
An absorption vessel independent of the ladle offers an additional solution. DE-OS 20 32 830 discloses a plunger, which is immersed with the open side down in the molten metal, and its inside is then evacuated. This plunger has the disadvantage that it must be pushed into the molten metal to achieve the required depth of immersion during the vacuum treatment. After the vacuum pressure is established, the level of the surface of the molten metal rises by the barometric differential, which can be much more than 1 meter, while the level of the molten metal not affected by the vacuum drops by a similar amount. As a result of the enclosure of molten metal in the plunger, which is smaller than the ladle, a relatively large volume of the molten metal is separated from the molten metal remaining in the ladle, with the disadvantage that the two portions of the molten metal are exposed to different vacuum treatments.
DE-AS 19 65 136 discloses an apparatus for the ladle degassing of molten metals, in which a reaction tube located underneath the cover of the vacuum treatment vessel can be immersed in the molten metal. In a complex manner, a lance with reactive gases is guided into the space enclosed by the reaction tube for the metallurgical treatment, during which the degassing and thus the volume increase of the molten metal is to take place. On account of the uniform vacuum pressure acting on the surface of the molten metal, it is not possible to prevent an increase in volume in the ring-shaped area between the reaction tube and the edge of the ladle.
DE-AS 19 12 907 and 19 19 053 also disclose devices in which gas is introduced into the molten metal through a tubular partition which is immersed in the molten metal. This partition is surrounded in a ring-shaped manner by an additional tubular partition, so that there is a communicating connection between these two partitions. By connection to pressure and/or suction pumps at different pressures, different surface level heights can be obtained in the individual chambers, and ultimately this leads to an improved flow of the metal or bath movement.